Having chemical stability and superior physical properties, microporous polyolefin film has been widely used as various battery separators, filters and ultrafiltration membranes.
The production of the microporous film by using polyolefin may be conducted according to the following three processes. According to the first process, polyolefin is processed into a thin fiber to produce a non-woven fabric-shaped microporous film. The second process is a dry process wherein a thick polyolefin film is prepared and stretched at low temperature to create micro-cracks between lamellas, the crystalline portion of polyolefin, to form micropores in polyolefin. The third process is a wet process wherein polyolefin is compounded with a diluent at high temperature to form a single phase, which is then subjected to phase separation into polyolefin and the diluent during the course of cooling, and the diluent portion is extracted to form pores in polyolefin. In comparison with the first or second process, the third process, that is a wet process, produces relatively thin microporous film with uniform thickness and excellent physical properties, so that the microporous film according to the wet process is widely used for a separator film of a secondary battery such as a lithium ion battery.
A method of preparing a porous film according to a wet process is disclosed in U.S. Pat. No. 4,247,498, which comprises blending polyethylene and a compatible liquid therewith at a high temperature to form a thermodynamically homogeneous solution, and cooling the solution to initiate solid/liquid or liquid/liquid phase separation, thereby producing a porous polyolefin film.
U.S. Pat. No. 4,335,193 discloses a method of preparing a porous polyolefin film, which comprises processing polyolefin by adding an organic liquid such as dioctylphthalate and liquid paraffin and inorganic filler; and removing the organic liquid compound and inorganic filler. Such a process is also described in U.S. Pat. No. 5,641,565. However, the process is disadvantageous in that it is difficult to feed and compound the inorganic filler because of employing inorganic fillers such as silica, and a subsequent process of extracting and removing the inorganic filler must be additionally conducted, thereby the process becomes complicated and difficult to increase the stretching ratio.
U.S. Pat. No. 4,539,256 also discloses a basic method of producing a microporous film by extruding polyethylene and compatible liquid compound, stretching and extracting the resultant mixture.
In conjunction with practical use of secondary batteries, efforts have been continuously made to improve the productivity and physical properties of a microporous film. A representative example is to improve the strength of the microporous film by using ultra-high molecular weight polyolefin (UHMWPO) with a weight average molecular weight of about 1,000,000, or blending such a UHMWPO to increase the molecular weight of the composition.
In this regard, U.S. Pat. Nos. 4,588,633 and 4,873,034 suggest a process for preparing a microporous film, wherein polyolefin having weight average molecular weight of 500,000 or more and a solvent capable of dissolving the polyolefin at high temperature are subjected to solvent extraction and stretching in two steps. However, this process is disadvantageous in that, in order to improve poor extrudability and compoundability of UHMWPO with the diluent (which are known as disadvantages of UHMWPO), an excessive amount of diluent is used in the extruding process, and the diluent must be extracted through two steps, before and after stretching.
U.S. Pat. No. 5,051,183 introduces a microporous polyolefin film, which includes a composition with polydispersity index (weight average molecular weight/number average molecular weight) of 10˜300, containing 10˜50 wt % of polyolefin having not less than 1% of UHMWPO with weight average molecular weight of 700,000 or more, and 90˜50 wt % of solvent such as mineral oil. In this regard, the composition is extruded to form a gel-like sheet and stretched at a temperature range from the melting point of the composition to a temperature higher than the melting point by 10° C., and the solvent is then extracted from the composition, thereby forming a porous film. However, this process is disadvantageous in that the molecular weight distribution is broadened upon blending with UHMWPO and the composition contains an excessive amount of polyolefin having high molecular weight. In such a case, severe chain entanglement occurs due to those molecules, resulting in significant reduction of stretchability. In other word, breakage occurs at a high stretching ratio and high speed, and non-uniform stretching occurs at a low stretching ratio.
In order to overcome those disadvantages, stretching temperature is increased to make the composition soft during the stretching process, or the stretching speed is reduced to afford the same effect as the raised temperature of the composition. In such a case, however, orientation of a resin becomes poor during the stretching process to lower the stretching effect, thereby deteriorating physical properties of the end porous film. Furthermore, a film made of the resin with broad molecular weight distribution has more defects due to the molecules with relatively low molecular weight, than a film made of the resin with narrow molecular weight distribution, thereby exhibiting reduced impact strength and puncture strength. Likewise, in the microporous film, the broad molecular weight distribution of polyethylene results in poor the puncture strength, which is one of the most important physical properties of the microporous film. In other word, UHMWPO incorporated into the microporous film does not contribute to improving physical properties thereof. Those disadvantages are also found in relevant prior arts, for example, in Japanese Patent Laid-Open Publication Nos. Hei 06-234876 and Hei 06-212006 and U.S. Pat. No. 5,786,396.
Meanwhile, Japanese Patent Laid-Open Publication No. Hei 09-3228 utilizes the similar composition as above, and suggests a method for improving physical properties of a microporous film by balancing stretching ratios in machine direction (MD) and transverse direction (TD).
Japanese Patent Laid-Open Publication No. Hei 09-259858 provides a process for preparing a microporous polyethylene film, which comprises preparing a solution including 10˜80 wt % of resin composition which contains 70˜99 wt % of polyethylene with weight average molecular weight of 500,000 or more and 1˜30 wt % of low molecular weight polyethylene with weight average molecular weight of 1,000˜4000, and 20˜90 wt % of solvent; extruding through a die and cooling the solution to form a gelated composition; stretching the gelated composition; and extracting the remaining solvent from the stretched composition, so as to reduce a shutdown temperature (at which the microporous film melts to block the pores to shut out electric current, and thus ignition or explosion is prevented when the temperature of a battery increases owing to an abnormal operation of the battery) of the microporous polyethylene film. In this respect, this process is characterized in that low molecular weight polyethylene with weight average molecular weight of 1,000˜4,000 is used to reduce the shutdown temperature of the microporous film. However, this process has two problems. First, use of molecules having low molecular weight brings about decrease in the molecular weight and broad molecular weight distribution, thereby deteriorating the physical properties of the microporous polyethylene film. From the examples of the patent, it can be seen that tensile strength of the microporous polyethylene film is relatively low as being 1,000˜1,200 kg/cm2. Second, a sophisticated technology is required to compound the polyolefin and diluent or solvent. For the commercial manufacturing, a twin screw extruder, a kneader, a Banbury mixer or the like is employed. In case that resins having significantly different viscosities (ultra-high molecular weight polyethylene with weight average molecular weight of 500,000 or more and low molecular weight polyethylene with weight average molecular weight of 1,000˜4,000) are blended with solvent, as described above, there are problems of compounding the resin and solvent, as well as compounding the two resins having different molecular weights (they have significantly different viscosities in molten state). In such a case, fine gels or fish eyes may occur on the end film, thereby deteriorating the quality of the film. To avoid such a phenomenon, there may be provided a method of increasing the residence time of the melt in an extruder, but the method causes disadvantage of decrease in productivity.
U.S. Pat. No. 5,830,554 recites a process for preparing microporous polyolefin film, in which a solution containing 5˜50 wt % of resin with weight average molecular weight of 500,000˜2,500,000 and a ratio of weight average molecular weight to number average molecular weight of 10 or less, is subjected to extruding, stretching and extracting processes in order to overcome the problems of poor physical properties and low stretchability resulted from the resin of broad molecular weight distribution. According to the method, a large amount (preferably, 80˜90 wt %) of solvent is used to avoid the problem of non-uniform extrusion caused by increased viscosity of the resin in the course of extruding the ultra-high molecular weight resin, and thus, the porosity is increased and the tensile strength of the porous film becomes 800 kg/cm2 or more (in examples, 950˜1,200 kg/cm2), which means that the physical properties of the porous film are not substantially improved.
Furthermore, U.S. Pat. No. 6,566,012 discloses a process for preparing a microporous film usefully applied to a battery separator, in which 10˜40 wt % of ultra-high molecular weight polyolefin with weight average molecular weight of 500,000 or more, or 10˜40 wt % of a resin composition containing ultra-high molecular weight polyolefin with weight average molecular weight of 500,000 or more, and 90˜60 wt % of solvent are subjected to extruding, molding, stretching, extracting and heat-setting steps.
As described above, the prior arts employ the resin with high molecular weight to improve the physical properties of the porous film, but the increase of molecular weight of the resin may bring about problems such as increased extrusion load, poor extrusion-compoundability of the resin with solvent, increased load of a stretcher during the stretching process, occurrence of non-uniform stretching, and decreased productivity owing to the decrease of stretching speed and ratio.
The present inventors have conducted extensive studies to avoid the above disadvantages resulted from the prior arts, and found that defects of polyethylene can be prevented from being formed and the stretching efficiency can be improved to provide sufficient properties as a separator by controlling the amount of low molecular weight polyethylene molecules and ultra-high molecular weight molecules contained in the polyethylene to a predetermined level or less and controlling the stretching condition, even by using polyethylene of moderate molecular weight without increasing the molecular weight of polyethylene by means of incorporating ultra-high molecular weight polyethylene, and completed the present invention.
Thus, the object of the present invention is to overcome the problems of prior arts involved with increased molecular weight, and provide a microporous high-density polyethylene film having excellent physical properties and uniform pore structure to be usable as a microporous film for a battery.
Another object of the present invention is to provide a process for economically preparing a microporous high-density polyethylene film with high productivity.